Paging storage is well known in the prior art and is provided by direct access devices (DASD) in current commercial data processing systems. Electronic storage paging devices have been suggested in the prior art, such as in an article in the IBM Technical Disclosure Bulletin Volume 23, No. 7A, December 1980, page 2908 entitled "Integrated Paging Storage" by J. T. Brady et al. Also, various means has been provided by the prior art to extend main storage by using special processor instructions to access an I/O device, such as a drum device in U.S. Pat. No. 2,796,218 (Tootill et al.) or U.S. Pat. No. 3,292,151 (Barnes et al.). Tootill used a central processor to control a block transfer between a drum and core storage. Automatic means (non-instruction) to control transfers in a hierarchical storage system is disclosed in U.S. Pat. No. 3,248,702 (Kilburn et al.) in which the program addresses the larger (and lower speed) storage in the hierarchy.
The prior art discloses the paging storage as being an I/O device which is handled on the system in the manner of any other I/O device, e.g. the IBM System/370 I/O control. In such prior system, data transfers by any I/O device (including the paging device) are between the I/O device and main storage under control of a channel program initiated by the central processor executing a start I/O (SIO) instruction. Such channel program therefore always handles the transfer between two media, i.e. the I/O device and main storage. These prior two-media channel programs are initiated by system control programs (e.g. OS/370 MVS auxiliary storage manager (ASM) and input output supervisor (IOS)) executed by the central processor (CP) to enable an application program to obtain the required paging and other I/O services, respectively.
In the prior art, DASD is used to provide both the paging storage and other I/O devices, and the same type of channel programming is used for both. They are distinguished only at the CP execution level. That is, pages in the paging storage DASD are recognized by the CP virtual addressing (i.e. segment and page tables) and are accessed implicitly by page faults caused during the normal CP execution of an application program. But, I/O DASD is not accessed by page faults in the system, and can only be accessed by an explicit I/O request in a CP program.
In the prior art, a transfer between a page storage DASD and an I/O DASD requires two separate two-media channel programs. One channel program controls the transfer between the I/O DASD and main storage, and the second channel program controls the transfer between main storage and the page storage DASD. Each channel program involves the normal large amount of preparatory system control program execution by the CP which culminates in executing a start I/O instruction that initiates the channel program which controls the transfer between the two-media. As a result, a three-media transfer is obtained between the page storage DASD medium and I/O DASD medium through the main storage medium using the two channel programs, which require the execution of two start I/O instructions, each being preceeded by the normal preparatory system control program execution.
In the prior art, all data transfers between the paging storage DASD and main storage, as well as between other I/O and main storage, pass through the channel on the channel processor data bus.